The present invention relates to an image transfer method for an image forming apparatus.
Among full-color image forming apparatuses, an electrophotographic printer using an intermediate image transfer body has an inherently high output speed and color reproducibility and is operable without regard to the kind of paper. This type of printer has a photoconductive element or similar image carrier made of a semiconductor or an insulator. An intermediate transfer belt or similar acceptor adjoins or contacts the image carrier in a preselected nip portion and moves in the same direction as the image carrier. The acceptor is implemented as a single semiconductor layer or provided with a double layer structure having a semiconductor layer and an insulator on the inner and outer peripheries thereof, respectively. Two conductors for applying transfer biases are respectively spaced apart from the middle point of the nip portion by distances L1 and L2 and located upstream and downstream with respect to the direction of movement of the acceptor. Toner or similar charged powder is conveyed by the image carrier to the nip portion between the two conductors. The charged powder is transferred from the image carrier to the acceptor a plurality of times to complete a full-color image. Specifically, latent images sequentially formed on the image carrier, or donor, are each developed by one of yellow, magenta, cyan and black toner to turn out a toner image. The toner images are sequentially transferred from the donor to the acceptor (primary transfer or intermediate transfer) one above the other, and then transferred from the acceptor to a paper at a time (secondary transfer).
However, the problem with the apparatus of the type described is that the image has its contour and colors blurred by so-called image dust. The image dust is particularly conspicuous when the primary transfer is repeated a plurality of times, and in this sense it is generally referred to as transfer dust. While various approaches to eliminate the transfer dust have been proposed in the past, none of them is satisfactory. Particularly, when the potential of the toner transferred to the acceptor remains at the time of the next primary transfer, the transfer dust is aggravated.